1. Field of the Invention
This invention relates to polyorganosiloxane-base copolymers of the crosslinked type and more particularly to films obtained therefrom. The films have selective gas permeability and improved mechanical strength.
2. Description of the Prior Art
Permeable, non-porous polymer membranes have particular utility in separating gas mixtures from each other to obtain the respective enriched gas components. In particular, oxygen which occupies 21 percent by volume of the air is one of the most important materials from the industrial standpoint particularly in the fields of general combustion furnaces, the iron-making industry, the food industry, medical appliances, waste treatments and the like. Accordingly, it is desired to separate oxygen from the air in an efficient, inexpensive and easy manner.
Typical of separation of oxygen or nitrogen from the air without use of any membranes are the PSA method and the low-temperature processing. The low-temperature processing requires a great deal of energy and is not economical, and the PSA method has the disadvantage that an oxygen- or nitrogen-enriched gas cannot be obtained continuously and is thus not produced on an industrial scale.
In contrast, the separation techniques using polymer membranes can continuously provide oxygen- or nitrogen-enriched gases and are thus very advantageous industrially. Accordingly, there is a strong demand for films or membranes which exhibit high selective separability and high permeability of intended gas. A number of methods using ultrathin or thin polymer members have hitherto been reported up to now. For separating oxygen or nitrogen from the air in amounts as much as possible by the use of thin polymer films, several important factors are known including the permeation coefficient of thin polymer film membranes with respect to oxygen or nitrogen, mechanical strength of the thin film, and thin film-forming techniques. Among polymeric materials, natural rubbers, synthetic rubbers such as polybutadiene, silicone rubbers and the like are known to have relatively excellent gas permeability, among which silicone rubbers are most excellent. That is, silicone rubbers show more excellent gas permeability than all other polymeric materials with respect to almost all gases though the permeability ratio of two gases becomes small, thus being considered as a practically convenient polymeric material. Silicone rubbers or polyorganosiloxanes have such properties that the interaction between molecules is small and the flexibility of the siloxane bonds is large. This is believed to be a reason why silicone rubbers are excellent in gas permeability. In this connection, the above properties are believed to have close relation with poor mechanical strength of silicone polymers: the small interaction between molecules causes a silicone polymer to be amorphous and thus lower its mechanical strength considerably. Accordingly, silicone polymers are utilized as permeable membranes after crosslinkage by curing into silicone rubbers. Silicone rubbers, when used as general structural materials, have very excellent weatherbility and satisfactory mechanical strength as is well known in the art, but are disadvantageous in that the cured silicone product is poor in film-forming property and is difficult to form into a thin film for gas permeation.
In order to overcome the above disadvantage and improve the film-forming property, an attempt has been made to provide block copolymers of, for example, polydimethylsiloxane-polycarbonate (U.S. Pat. No. 3,767,737). These block copolymers have sufficient mechanical strength to be formed into thin films without undergoing curing treatments and are soluble in organic solvents, so that film formation is feasible by casting or other ordinary techniques, thus providing thin films readily.
However, because of the introduction of other polymers to the polydimethylsiloxane, the block copolymers have a relatively low siloxane content and the gas permeability is lowered to a level of about 1/3 times that of silicone polymer itself. From this, it is considered that if copolymers having an increasing content of siloxane and sufficient mechanical strength for film formation are obtained, gas-permeable films having improved gas permeability over the block copolymers will be formed.